Despite its importance as the source of one of three major vascular systems in the mammalian conceptus, little is known about the murine allantois, which will become the umbilical cord of the chorio-allantoic placenta. During gastrulation, the allantois grows into the exocoelomic cavity as a mesodermal extension of the posterior primitive streak. On the basis of morphology, gene expression and/or function, three cell types have been identified in the allantois: an outer layer of mesothelial cells, whose distal portion will become transformed into chorio-adhesive cells, and endothelial cells within the core.Formation of endothelium and chorio-adhesive cells begins in the distal region of the allantois, farthest from the streak. Over time, endothelium spreads to the proximal allantoic region, whilst the distal outer layer of presumptive mesothelium gradually acquires vascular cell adhesion molecule(VCAM1) and mediates chorio-allantoic union. Intriguingly, the VCAM1 domain does not extend into the proximal allantoic region. How these three allantoic cell types are established is not known, although contact with the chorion has been discounted.In this study, we have investigated how the allantois differentiates, with the goal of discriminating between extrinsic mechanisms involving the primitive streak and an intrinsic role for the allantois itself. Exploiting previous observations that the streak contributes mesoderm to the allantois throughout the latter's early development, microsurgery was used to remove allantoises at ten developmental stages. Subsequent whole embryo culture of operated conceptuses resulted in the formation of regenerated allantoises at all time points. Aside from being generally shorter than normal, none of the regenerates exhibited abnormal differentiation or inappropriate cell relationships. Rather, all of them resembled intact allantoises by morphological, molecular and functional criteria. Moreover, fate mapping adjacent yolk sac and amniotic mesoderm revealed that these tissues and their associated bone morphogenetic protein 4 (BMP4) did not contribute to restoration of allantoic outgrowth and differentiation during allantoic regeneration.Thus, on the basis of these observations, we conclude that specification of allantoic endothelium, mesothelium and chorio-adhesive cells does not occur by a streak-related mechanism during the time that proximal epiblast travels through it and is transformed into allantoic mesoderm. Rather, all three cell-types are established by mechanisms intrinsic to the allantois, and possibly include roles for cell age and cell position. However, although chorio-adhesive cells were not specified within the streak, we discovered that the streak nonetheless plays a role in establishing VCAM1's expression domain,which typically began and was thereafter maintained at a defined distance from the primitive streak. When allantoises were removed from contact with the streak, normally VCAM1-negative proximal allantoic regions acquired VCAM1. These results suggested that the streak suppresses formation of chorio-adhesive cells in allantoic mesoderm closest to it.Together with previous results, findings presented here suggest a model of differentiation of allantoic mesoderm that invokes intrinsic and extrinsic mechanisms, all of which appear to be activated once the allantoic bud has formed.